1. Field of the Invention
The present invention relates to the field of still video imaging, and especially to the operation of an image sensor in a camera capable of producing a single-field recording.
2. Description Relative to the Prior Art
A new video recording format has been introduced in which a small flexible magnetic disk is used to store up to 50 still video pictures. This format provides the option of storing a full frame (two circular tracks per picture) for 25 pictures, or a single field (one circular track per picture) for 50 pictures. Though single field recording inevitably limits playback resolution compared to full-frame recording, a satisfactory "full-frame" result can be played back by interpolating a second field from successive lines of the recorded (first) field. U.S. Pat. No. 4,470,076 illustrates such interpolation with a still image obtained from a limited-resolution, single-field image sensor. The principal attractiveness of single field recording is the higher picture density obtained on the recorded disk. Nonetheless, frame recording provides an inherently better result. A versatile video camera would therefore offer the user both capabilities: frame and field recording.
Frame recording requires that color information regarding both fields be simultaneously generated within an image sensor capable of separately resolving, and outputting, each field. Without simultaneously-generated fields, any movement between fields would provide an unsightly image. Since the aforementioned still format provides line sequential color, each line of the image sensor must separately provide the necessary primary color signals (R, G or B) for generating a line sequential color difference (R-Y or B-Y). A typical sensor of this type employs an interline-transfer technique. In such a sensor, photo-generated charges stored in the odd horizontal lines are read out for the odd video field, while the other line charges are read out for the even field. Such sensor operation can be compared with a video (movie) operating method called "frame integration", in which photo-generated electrons in each photoelectric element are accumulated for one television frame time, i.e., 1/30 second in the NTSC system. In a still video camera, however, the photo-generated electrons in each photoelectric element are accumulated for a variable period dependent upon the required still exposure.
Significantly, line charges must be maintained distinct so that one field is unaffected by charges from the other field. In this way, the fields are temporarlly separable though they have been concurrently integrated. (Another movie mode, referred to as "field integration", requires the addition of line charges in two adjacent lines in order to generate the primary color signals. Field integration is unsuitable for full-frame still imaging because it depends on a successive generation of two fields to provide a frame. In other words, each field "uses up" all the charge accumulated by the sensor and the sensor must be re-exposed for the second field. For a discussion of both field and frame integration, see "Field Integration Mode CCD Color Television Camera Using a Frequency Interleaving Method", by Koshinori Takizawa et al, IEEE Trans. on Consumer Electronics, Vol. CE-29, No. 3, August 1983, pp. 358-364.)
In a still video camera having a full-frame interline-transfer image sensor with concurrent field integration, the provision for single field recording uses only half of the photoelectric elements of the sensor. Drawing an analogy to conventional photographic film, the effective "resolution" of the sensor is reduced accordingly. With film, however, resolution can be traded for "film speed", that is, lowering resolution ordinarily leads to an increase in photosensitivity. This does not happen with an electronic sensor operating in a field mode because the light sensitivity of the sensor is dependent upon half the photoelectric elements in half the area of the sensor. No benefit in "speed" is obtained.
U.S. Pat. No. 4,603,354 discloses an electronic camera having an image sensor operable in either a still frame mode or a still field mode, in both cases using each field. Odd and even fields are produced and read out separately in the frame mode. The field mode, on the other hand, provides a "merged" output by the addition of the information charges of both fields. For the merged field, the information charge put out by the image sensor is double that put out for each field in the frame mode--each image location has twice as much charge. As a result, since the information charge in the frame and merged field modes is different, either attenuating or amplifying means are provided for adjusting the level of the signal from the image sensor. In this way, the two modes provide equal output. This level adjustment, however, provides two unattractive alternatives: either increase the gain of the frame signal, thereby adding noise to the supposedly "high quality" signal, or decrease the gain of the merged field signal, thereby reducing effective photosensitivity in the potentially "high speed" signal.